1. Field of the Invention
The present invention relates to selectable resolution in a CCD imaging sensor. In particular, the invention relates to use of a dual function clock conductor in the clocking structure to mechanize a two dimensional selectable resolution feature in area and TDI sensors and a one dimensional selectable resolution feature in the readout register of the sensor and in one dimensional linear sensors.
2. Description of Related Art
Charge coupled devices (hereinafter CCDs) are widely used in video imaging and recording applications. For example, the architecture of a CCD video sensor may follow the form dictated by the National Television Standards Committee (NTSC) for video broadcast standards. Such CCD video sensor designs need at least 488 TV lines vertically, 500 to 800 pixels per TV line, have an optical format of 4/3 aspect ratio, and generate field interlaced video at a frame rate of 30 Hz. CCD architectures which achieved the goals of the video format imaging requirements generally fall into two categories: Interline Transfer (ILT) or Frame Transfer (FT) image sensors.
An alternative application of a CCD sensor is industrial inspection or vision equipment. The architecture of a CCD video sensor for this application may be optimized for maximum pixel resolution, or to maximize image frame rate, or both. Often inspection cameras used to inspect moving objects (e.g., on a continuous conveyor belt or rolled goods such as rolls of cloth) employ a line scan CCD sensor where a linear CCD sensor is oriented in a direction perpendicular to the direction of movement of the object being imaged. Advanced linear CCD sensors often employ a time delay and integrate technology and are referred to as TDI CCD sensors.
In FIG. 7, known frame transfer sensor 10 includes imaging section 2 coupled to storage section 4. Frame transfer sensors rely on the quick transfer of image field data from an array of photo-sensitive photo-sites in imaging section 2 where photo-charge is integrated (i.e., the imaging section) to an optically opaque analog storage array (i.e., storage section 4), and the subsequent parallel to serial transfer of the video data through horizontal CCD readout shift register 6 (HCCD shift register) through output node structure 7, through buffer 8 to a camera circuit. An optically opaque storage array is a storage array that is covered by an optically opaque material such as an aluminum film so that the storage array is not photo-sensitive. The rapid vertical transfer of the image data from integration of the first video field is vital since the optical input onto the imaging section of the CCD sensor is continuous (not strobed or modulated during transfer).
In FIG. 8, known interline transfer CCD image sensor 20 (i.e., an ILT CCD sensor) includes imaging section 12 in a plurality of columns of photo-sites, each column disposed adjacent to an optically opaque interline transfer register 14 (e.g., covered with aluminum to render the transfer register insensitive to optical input). An ILT CCD sensor generally has no optically opaque storage section as does the frame transfer sensor. In an ILT CCD sensor, optical input is integrated in photo-sites 12 and then transferred to interline transfer registers 14. Then, during the integration of the next field of data, the field of image data in interline transfer registers 14 are parallel to serial transferred through horizontal CCD shift register 16 (HCCD shift register) through output node structure 17, through buffer 18 to a camera circuit.
In FIG. 9, known time delay and integrate (TDI) linear array sensor 30 includes imaging section 22 as in a frame transfer sensor, but the imaging section is generally coupled directly to horizontal CCD readout shift register 26, and from there through output node structure 27, through buffer 28 to a camera circuit. Imaging section 22 includes a plurality of columns, each column including a plurality of photo-sites. In operation, a camera lens focuses the image on the TDI CCD sensor. The image, that is the optical input to the TDI CCD sensor, is moving (e.g., on a conveyor belt). An image conjugate focused on the sensor appears to be moving. A portion of the image first appears on one pixel of the TDI CCD sensor, and then appears on another pixel of the sensor. The camera and sensor are arranged so that a portion of the moving image moves in a direction from the top of a column of photo-sites to the bottom of the column. The TDI CCD sensor is clocked to transfer charge down the columns of photo-sites at a rate equal to the rate that the portion of the image moves down the column. Charge generated at a first photo-site is transferred to the next photo-site at the same time that the image portion that generated the charge at the first photo-site moves to the next photo-site. In this way photo-charge is accumulated at the photo-site under the image portion as the image portion moves down the column. Thus the name, time delay and integrate (TDI).
Two-dimensional imaging arrays (e.g., in frame transfer sensors and in interline transfer sensors) generally take a snap shot of an image. The whole photo-active array of pixels integrates photo-charge for a period of time. At the end of the integration time, the information is transferred from the sensor to an external circuit element using a parallel to serial transfer scheme. Each horizontal line of pixel data is transferred into a horizontal CCD readout shift register. The line of data is then transferred serially through an output node structure at the end of the register and then through a buffer amplifier.
Similarly, in a TDI CCD sensor, the last pixel in each integrating column of photo-sites (i.e., the last horizontal line) is transferred into a horizontal CCD readout shift register. The line of data is then transferred serially through an output node structure at the end of the register and then through a buffer amplifier.
In TDI CCD sensors, it is sometimes desirable to adjust the responsivity of the sensor to the brightness of the light illuminating the scene being imaged. In applications of two dimensional imaging arrays (e.g., in frame transfer sensors and in interline transfer sensors), it is sometimes desirable to provide a reduced format sensor (i.e., only the lower half or quarter of the image area is output). In applications of two-imensional imaging arrays (e.g., in frame transfer sensors and in interline transfer sensors) or in a TDI sensor, it is sometimes desirable to provide a selectable resolution in the horizontal direction (i.e., only the right or left half or quarter of the image area is output). In one dimensional line scan applications, it is sometimes desirable to provide a one dimensional selectable resolution in the horizontal direction (i.e., only the right or left half or quarter of the line array is output).